This application is being filed concurrently with related U.S. Patent Applications: xe2x80x9cBISTABLE MICROMIRROR WITH CONTACTLESS STOPSxe2x80x9d by Lilac Muller; xe2x80x9cMETHODS AND APPARATUS FOR PROVIDING A MULTI-STOP MICROMIRROR,xe2x80x9d by David Paul Anderson; and xe2x80x9cSYSTEMS AND METHODS FOR OVERCOMING STICTION USING A LEVER,xe2x80x9d by Bevan Staple, David Paul Anderson, and Lilac Muller; all of which are herein incorporated by reference in its entirety for all purposes.
This application relates generally to microelectromechanical systems, and more particularly to MEMS devices and methods configured to avoid stiction.
In recent years, increasing emphasis has been made on the development of techniques for producing microscopic systems that may be tailored to have specifically desired electrical and/or mechanical properties. Such systems are generically described as microelectromechanical systems (MEMS) and are desirable because they may be constructed with considerable versatility despite their very small size. In a variety of applications, MEMS component structures may be fabricated to move in such a fashion that there is a risk of stiction between that component structure and some other aspect of the system. One such example of a MEMS component structure is a micromirror, which is generally configured to reflect light from at least two positions. Such micromirrors find numerous applications, including as parts of optical switches, display devices, and signal modulators, among others.
In many applications, such as may be used in fiber-optics applications, such MEMS-based devices may include hundreds or even thousands of micromirrors arranged as an array. Within such an array, each of the micromirrors should be accurately aligned with both a target and a source. Such alignment is generally complex and typically involves fixing the location of the MEMS device relative to a number of sources and targets. If any of the micromirrors is not positioned correctly in the alignment process and/or the MEMS device is moved from the aligned position, the MEMS device will not function properly.
In part to reduce the complexity of alignment, some MEMS devices provide for individual movement of each of the micromirrors. An example is provided in FIGS. 1A-1C illustrating a particular MEMS micromirror structure that may take three positions. Each micromirror includes a reflective surface 116 mounted on a micromirror structural film 112 that is connected by a structural linkage 108 to an underlying substrate 104. Movement of an individual micromirror is controlled by energizing actuators 124a and/or 124b disposed underneath the micromirror on opposite sides of the structural linkage 108. Hard stops 120a and 120b are provided to stop the action of the micromirror structural film 112. Energizing the actuator 124a on the left side of the structural linkage 108 causes the micromirror to tilt on the structural linkage 108 towards that side until one edge of the micromirror structural film 112 contacts the left hard stop 120a, as shown in FIG. 1A. Alternatively, the actuator 124b on the right side of the structural linkage 108 may be energized to cause the micromirror to tilt in the opposite direction, as shown in FIG. 1B. When both actuators are de-energized, as shown in FIG. 1C, the micromirror returns to a static position horizontal to the structural linkage 108. In this way, the micromirror may be moved to any of three positions. This ability to move the micromirror provides a degree of flexibility useful in aligning the MEMS device, although the alignment complexity remains significant. Sometimes hard stops 120a and 120b are not provided so that the micromirror structural film 112 is in direct contact with the substrate 104.
In certain applications, once the micromirror is moved to the proper position, it may remain in that position for ten years or more. Thus, for example, one side of an individual micromirror structural film may remain in contact with the hard stop or substrate for extended periods. Maintaining such contact increases the incidence of dormancy-related stiction. Such stiction results in the micromirror remaining in a tilted position even after the actuators are de-energized. Some theorize that stiction is a result of molecule and/or charge build up at the junction between the micromirror structural film and the hard stop or substrate. For example, it has been demonstrated that an accumulation of H2O molecules at the junction produces capillary forces that increase the incidence of stiction.
Thus, one solution to overcome stiction is to package the MEMS device in a hermetic or inert environment. Such an environment reduces the possibility of molecule accumulation at the junction. However, such packaging is costly and prone to failure where seals break or are not properly formed. Further, such packaging is incompatible with many types of MEMS devices. In addition, such packaging does not reduce stiction related to charge build up at the junction.
In xe2x80x9cUltrasonic Actuation for MEMS Dormancy-Related Stiction Reductionxe2x80x9d, Proceedings of SPIE Vol. 4180 (2000), which is herein incorporated by reference for all purposes, Ville Kaajakari et al. describe a system for overcoming both molecule and charge related stiction. The system operates by periodically vibrating an entire MEMS device to overcome stiction forces. While there is evidence that vibrating the entire MEMS device can overcome stiction at discrete locations within the device, such vibration causes temporary or even permanent misalignment of the device. Thus, freeing an individual micromirror often requires performance of a costly alignment procedure. Even where the device is not permanently misaligned by the vibration, it is temporarily dysfunctional while the vibration is occurring.
Thus, there exists a need in the art for systems and methods for overcoming stiction in MEMS devices without causing misalignment.
Embodiments of the invention are directed to a microstructure for steering light that mitigates stiction problems. A first tiltable assembly that includes a reflective coating is connected with a substrate. A second tiltable assembly is also connected with the substrate. First and second electrodes are connected with the substrate and are configured to tilt the two tiltable assemblies such that they are interdigitated. In various embodiments, the tiltable assemblies are configured as cantilever arrangements and/or torsion-beam arrangements.
The first tiltable assembly may include a first structural linkage connected with the substrate. A first structural film is supported by the first structural linkage and has a plurality of fingers at an end of the first structural film. The reflective coating is included on the first structural film. The second tiltable assembly may similarly include a second structural linkage connected with the substrate. A second structural film is supported by the second structural linkage and has a plurality of fingers at an end of the second structural film. The fingers at the ends of the first and second structural films may be used to interdigitate the tiltable assemblies. In one embodiment, the first structural linkage has a greater height above the substrate than the second structural linkage.
Embodiments of the invention are also directed to a method for fabricating such a microstructure. A first tiltable assembly that includes a reflective coating is formed on a substrate. A second tiltable assembly is also formed on the substrate. First and second electrodes are formed on the substrate and are configured to tilt the tiltable assemblies upon activation such that the tiltable assemblies interdigitate.
In still other embodiments, a method is provided for operating an optical switch. A first assembly is tilted by applying a first electrostatic force. The first assembly may include a first pivot connected with a substrate, a first structural film supported by the first structural linkage, and a reflective coating. The first structural film may have a plurality of fingers at an end of the first structural film. A second assembly is also tilted by applying a second electrostatic force. The second assembly may include a second pivot connected with the substrate and a second structural film supported by the second structural linkage. The second structural film also includes a plurality of fingers at an end of the second structural film. The first and second assemblies are held electrostatically in a fixed position with the fingers of the first and second structural films interdigitated.
In still other embodiments, a wavelength router that incorporates a microstructure for steering light is provided. The wavelength router is configured for receiving light having a plurality of spectral bands at an input port and for directing subsets of the spectral bands to a plurality of output ports. A free-space optical train is disposed between the input port and the output ports providing optical paths for routing the spectral bands. The optical train also includes a dispersive element disposed to intercept light traveling from the input port. A routing mechanism is provided having at least one dynamically configurable routing element to direct a given spectral band to different output ports. The dynamically configurable routing element includes a tiltable micromirror assembly having a micromirror structural film with a plurality of fingers at one of its ends. It also includes a tiltable snare assembly having a snare structural film with a plurality of fingers at one of its ends. A plurality of electrodes is configured to tilt the micromirror assembly and snare assembly upon activation such that the fingers of the micromirror structural film and snare structural film interdigitate.